One of the problems you'll run into with blocking IPs is that not everyone has a static IP address. It's more common to get a different IP address every time you access your ISP. If you start getting hit by a troll, and you know their IP address you could block the specific IP address, but if they come back then you'll have to block a range of addresses.
With UBB Classic I use to block registrations that used an email address from one of the anonymizer sites, and at one time I had to block AOL and Yahoo email addresses...
